The present invention relates to a porous film, a process for producing the same and its use. More particularly, the present invention relates to a porous film which is excellent in latent heat resistance or heat resistance, a process for producing the same and its use.
Conventionally, various types of batteries have been actually used. Recently, in order to meet the needs for making electronic devices cordless, or the like, a lithium battery, which has a light weight, is capable of obtaining a high electromotive force and high energy, and has low self-discharge, has been sought. For instance, lithium secondary batteries have been used in large amounts for cellular phones and notebook computers, and they are further expected to be used for batteries for electric automobiles in the future.
In general, in the lithium battery described above, there is provided a separator between a positive electrode and a negative electrode, the separator functioning to prevent short-circuit between these electrodes. As the separator mentioned above, a porous film having a large number of micropores has been usually used for securing transmission of ions existing between the positive electrode and negative electrode.
As the separator for batteries mentioned above, there have been proposed various porous films each comprising an ultra-high molecular polyolefin resin and additionally another polyolefm resin. For instance, Japanese Examined Patent Publication No. Hei 6-53826 discloses a porous film comprising an ultra-high molecular polyethylene resin having a weight-average molecular weight of 5xc3x97105 or more.
However, in order to actually use these porous films, each comprising a polyolefin resin for a separator for batteries, especially for batteries for electric automobiles, their heat resistance cannot be said to be sufficient.
In addition, there has been proposed a porous film further comprising a rubber in addition to the polyolefin resin. For instance, Japanese Examined Patent Publication No. Hei 1-18091 discloses a porous film formulated with a liquid rubber for the purposes in the improvements in stretchability, flexibility, tear strength, moisture permeability, and the like. Japanese Patent Laid-Open No. Hei 9-169867 discloses a microporous film added with a hydrogenated styrene-butadiene block copolymer for the purposes of giving uniform structure and high puncture strength. In addition, Japanese Patent Laid-Open Nos. Hei 6-163023 and Hei 8-138644 each discloses a battery separator comprising ethylene-propylene rubber, for the purposes of imparting a balance in the mechanical strength and improving the impregnation ability of electrolyte.
However, none of these inventions are designed to improve heat resistance, and they neither disclose nor suggest heat resistant properties.
The problems seen in the actual use of a battery, such as in the case of the lithium battery, include the possibility that when the battery is allowed to stand in an unusually high temperature, or over-charged, or when the short-circuit is caused between the electrodes in the internal or external of the battery, the battery generates an unusual amount of heat, so that an electrolyte in the internal of the battery is discharged to the external of the battery by their dramatic temperature spike. Although various safety devices for temperature spikes are assembled in a single battery cell or a group of battery cells, their effectivity is low for a dramatic temperature spike when partially short-circuited, or the like.
Causations for such unusually high temperature spikes include external short-circuiting, forced discharging, over-charge, precipitation of dendrites, internal short-circuiting caused by penetration of battery with a projected object (so called xe2x80x9cnail-penetratingxe2x80x9d), allowing to stand in an unusually high temperature, and the like.
In addition, in the case of a battery for electric automobiles, single battery cells are subject to high-output, and such battery cells are used in a large number, so that there is a large risk that unusual heat generation takes place. Especially, when automobile accident and the like are considered, there cannot be completely eliminated unpredictable unusual factors such as surrounding temperature increase caused by short-circuiting by battery penetration or the like. In such a case, when an unusual heat generation takes place in one of the battery cells, there arises a risk of causing secondary and tertiary casualties. Since a large number of battery cells are collectively used in electric automobiles, the safety for individual battery cells would become increasingly important.
In view of these circumstances, a separator primarily having a function of preventing short-circuiting between the electrodes has been expected to have excellent heat resistance, so that the short-circuiting can be prevented even under unusually high temperatures. In addition, as described above, a wide range of short-circuiting can be prevented between the electrodes, if the separator has a sufficient heat resistance not to be melted, deformed, or broken up to high temperatures, so that effects of preventing casualties caused by unusual heat generation of batteries and the like can be expected.
An object of the present invention is to provide a porous film having appropriate porosity, air permeability and puncture strength, and being especially excellent in latent heat resistance or heat resistance, and a process for producing the same; further a separator for a battery comprising the porous film; and a battery comprising the separator.
As a result of intensive studies in order to solve the problems mentioned above, the present inventors have found that a porous film comprising a polymer having Cxe2x95x90C double bond and an aliphatic ring in its main chain, and another resin exhibits excellent latent heat resistance or heat resistance. The present invention has been accomplished thereby.
Specifically, the present invention relates to:
(1) a porous film comprising a polymer composition comprising (a) a polymer having Cxe2x95x90C double bond and an aliphatic ring having 5 to 10 carbon atoms in a main chain thereof; and (b) another resin;
(2) a porous film comprising the (a) polymer and the (b) resin of item (1) above, all or a part of Cxe2x95x90C double bond in the (a) polymer being disappeared;
(3) a porous film comprising the (a) polymer and the (b) resin of item (1) above, wherein the (a) polymer and the (b) resin are cross-linked therebetween;
(4) a process for preparing the porous film of item (2) or (3) above, characterized by heat-treating the porous film of item (1) above in the presence of oxygen;
(5) a separator for a battery comprising the porous film of any one of items (1) to (3) above; and
(6) a battery comprising the separator of item (5) above.